This paper presents the three-year impacts of an improved biomass cookstove on child and adult health in rural Ethiopia. After near complete stove adoption during an initial one-year randomized controlled trial, 60 percent of treatment households continued to use the improved stoves three-years on and experienced reductions in hazardous airborne particulate matter. The study finds that treatment status is associated with a precisely estimated 0.3-0.4 standard deviation improvement in height-for-age of young children exposed during their first years of life, compared with a control group of households that never used the improved stove. This is a substantial effect with implications for greater health and well-being throughout the life course. However, the study finds no changes in the respiratory symptoms or physical functioning of older children and adult cooks in treated households relative to control households. The results advance understanding of the health impacts of hazardous air pollution while also refining the design and implementation options for interventions geared toward improving well-being in similar environments.